Paints to give metallic (brilliant) coated films have been used in coating in a volume of ratios for automobile exterior plates and parts thereof (e.g., door mirror, wheel cover, and front grille) and motorcycles, and also recently for OA devices, such as personal computer and word processor, cell phones, and home appliances, such as cabinet. Such a paint should form a colored metallic film classy in appearance and different in color, and metallic basecoat paints containing a scale brilliant pigment (such as aluminum pigment or mica pigment) have been developed. The metallic coated film obtained by using the above paint appears brilliant by reflection of external incident light by the brilliant pigment contained in the coated film, and the coated films in various colors provide characteristic decorative appearance rich in variety.
As a method of forming a metallic coated film, JP-B-2-38267 (“JP-B” means examined Japanese patent publication) discloses, for example, a metallic painting method of forming a metallic base film containing a relatively deep-colored pigment, and further forming thereon a colored clear coated film containing, at a low concentration, a high-chroma pigment similar in color to the relatively deep colored pigment. Further, WO97/47396 discloses a method of forming a metallic coated film, by sequentially applying a metallic base paint, a second base paint having transparency, and a clear paint.
However, in the method described in JP-B-2-38267, the relatively deep colored pigment contained in the metallic base film deteriorates metallic appearance, and thus, the resultant metallic coated film is not satisfactory in color depth and high color saturation. Further, in the method described in WO97/47396, the color of the coated film varies largely, due to slight change in coated film thickness of the second base paint having transparency, making it difficult to control the painting line and consequently, thus being apt to lead so-called framing phenomena, such as increase in film thickness and darkening of color in the edge regions.
It is possible to improve the above-mentioned phenomena drastically, by using a highly transparent dye as a colorant (a material for coloring the paint). However, the kinds of dyes usable in the paint, such as chelate dye (metal-containing dye) containing heavy metals such as chromium, are limited, from the viewpoint of weather resistance. Further, the metal-containing dyes are still lower in weather resistance than pigments, and the heavy metals such as chromium contained are environmentally hazardous, and thus, there exists a need for development of a novel colorant high in transparency. If such a colorant can be prepared with a pigment, it likely satisfies the requirements above. Thus, for providing a colorant which satisfies the aforementioned requirements, it is essential to produce a stable pigment dispersion high in transparency and resistant to aggregation even at higher concentration.
For preparation of a transparent pigment dispersion, it is necessary to produce a dispersion of pigment fine-particles small in average particle diameter and narrow in particle size distribution. Proposed for production of such a dispersion is a build-up method of producing particles chemically, but not a breakdown method of producing particles by pulverization of pigment. One of the methods is a production method of producing stable fine-particles, by precipitating an organic pigment by bringing a solution in which the organic pigment has been dissolved into contact gradually with an aqueous medium, in which a dispersant is added to one of the liquids (see JP-A-2003-26972 (“JP-A” means unexamined published Japanese patent application), JP-A-2003-113341, JP-A-2003-128955, and JP-A-2004-43776). Further, methods of performing the operation in a microreactor are also disclosed (see JP-A-2005-307154 and JP-A-2006-104448). Further, recently disclosed is a method of producing an organic pigment dispersion improved in the dispersion stability, by precipitating pigment fine-particles in the presence of a polymerizable compound, and then polymerizing the polymerizable compound (see JP-A-2007-39643).
However, the pigment concentration of the conventional pigment fine-particle dispersions is low, and increase in that concentration causes problems such as drastic increase in viscosity and deterioration in transparency.
Generally, glass products are often used as colored, for example, in applications, such as color filter for liquid crystal display devices, colored glass bottle, and others. One of the methods of obtaining a colored glass product, such as color filter or colored bottle, is a so-called sol-gel process of blending an organic colorant (e.g., dye or pigment) with silica sol, applying the resultant mixture on a glass surface, and baking the film for gelling. Generally, in the sol-gel process, a dense colored film cannot be obtained without heat treatment at high temperature. Use of a dye as the organic colorant often gives a colored film favorably transparent with low haze. However, dyes are often decomposed at high temperature, and thus, it is not possible to raise the baking temperature, which leads to deterioration in film density.
To overcome the problem, disclosed is a method of forming a color filter, by mixing organic pigment fine-particles having a small particle size, for example, of 3 nm to 300 nm with a metal alkoxide by use of a dispersant, applying the resultant mixture on a glass surface, and baking the coated film (see JP-A-5-178623). However, it is difficult to obtain fine particles having a uniform particle diameter by conventional breakdown methods based on mill dispersion, and, even if possible, such a method demands a vast amount of labor. In particular, it is quite difficult to obtain uniform fine particles having a particle diameter of 100 nm or less, which is essential for transparency.
To improve that point, disclosed is a method of using a solution containing a metal alkoxide and/or a metal-alkoxide hydrolytic condensate and a latent pigment (see JP-A-2001-207115). The colored film obtained by the method is more transparent than those obtained by conventional methods. However, it is a method of generating a pigment by high-temperature pyrolysis after coating, and thus, an unevenly colored film is often formed when a film of large area is prepared, due to irregularity of temperature distribution. Generally, gases such as carbon dioxide are generated, and for that reason, improvement in film density is still needed.
As described above, it is difficult to obtain a highly transparent colored film containing a dispersed pigment, and the same is true when a colored glass or a colored glossy-surfaced metal plate is prepared. Thus, there is a need for development of a colored coating composition that gives a colored film excellent in uniformity and transparency, that has desired film physical properties, and that can be used in various application uses.